1. Field of the Invention
This invention relates to a solid-state image pickup device for use in a television camera, etc. Particularly it relates to a solid-state imaging device which has a plurality of picture elements and horizontal and vertical scanning circuits, which are disposed in a major surface region of a semiconductor body.
More specifically, it relates to a solid-state device which has picture elements for reading out from photodiodes optical information stored therein.
2. Description of the Prior Art
A solid-state imaging device for use in a television camera must possess a resolution equivalent to that of an image pickup tube used in present day television broadcasting. Therefore, it requires about 500.times.500 photoelectric conversion elements, switching transistors for X-Y addressing which correspond to the photoelectric conversion elements, and an X-scanner and a Y-scanner which turn the switching transistors "on" and "off" and each of which consists of about 500 stages. Accordingly, the solid-state image pickup device is usually fabricated by MOS LSI technology which can realize a high integration density comparatively easily.
FIG. 1 is a diagram for explaining the configuration of such a solid-state imaging device. Referring to the figure, numeral 1 designates a horizontal scanning circuit for X or column addressing, while numeral 2 designates a vertical scanning circuit for Y or row addressing. Numeral 3 designates a vertical switching transistor which is turned "on" or "off" by the circuit 2 and which is formed of a metal-oxide-semiconductor field effect transistor (hereinbelow, abbreviated to "MOST"). Numeral 4 designates a photodiode which employs the source junction of the switching transistor 3. Shown at 5 is a vertical output line to which the drains of the switching transistors 3 are connected in common. Numeral 6 indicates a horizontal switching transistor which is turned "on" or "off" by the horizontal scanning circuit. It is formed of a MOST, the drain of which is connected to a horizontal output line 7 and the source of which is connected to the vertical output line 5. A video voltage source 8 is connected to the horizontal output line 7 through a resistor 9. The two horizontal and vertical scanning circuits turn the switching transistors 6 and 3 "on" and "off" in succession, to read out an image or video signal through the resistor 9 comprised of photo-currents from the photodiodes which are arrayed in two dimensions. Since the signals from the photodiodes correspond to the optical image of an object projected thereon, a video signal can be derived by the above operation. A feature of this type of solid-state imaging device is its easy integration since the sources of the switching MOSTs can be utilized for photoelectric conversion and MOS shift registers can also be utilized for scanning circuits.
The solid-state imaging device above described, however, involves the problem that charges generated by photo-excitation in the region of intense incident light overflow to surrounding regions with the result that white spots reflecting diffusion spread on a monitor and that a white vertical stripe appears. The white spot of the former corresponds to a case where overflow charges have diffused into the adjacent photodiode, while the vertical stripe of the latter corresponds to a case where the charges have diffused into the vertical output line. The vertical stripe is a reproduced image which is not included in the actual optical image, and hence, it gives an unnatural impression. This phenomenon is generally called "blooming," and is a serious factor hampering the practicability of the solid-state imaging device. For example, the device cannot be used in natural daylight where the intensity of incident light is high. Even in the interior of a house, the blooming phenomenon occurs when the images of a metal having high reflection factor, a white object, etc. are picked up. Therefore, the places of use are limited severely.
As a result of empirical measurements with an area sensor, the inventors have found that the blooming is attributed to a cause as described hereunder. FIG. 2A shows the sectional structure of a picture element which forms a unit of the construction shown in FIG. 1. Numeral 10 designates a semiconductor body of a first conductivity type (e.g., P-type conductivity) in which the elements are integrated. Numeral 11 designates a vertical switching MOST which is formed of an insulating film 12, a gate electrode 13, a source 14 and a drain 15. The source and drain are made of diffusion layers of an impurity of a second conductivity type (e.g., N-type conductivity). The source junction (NP-junction) is utilized as a photodiode. The drain is connected to an aluminum (Al) interconnection 16 for a vertical output line.
FIGS. 2B, 2C and 2D are energy band diagrams corresponding to FIG. 2A. In the diagrams, "C.B." indicates electrons or signifies the conduction band, and "V.B." designates holes or signifies the valence band.
When the switching MOST 11 is rendered conductive by a pulse of level "1" provided by the vertical scanner, the photodiode 14 is charged up to a video voltage Vv by the video voltage source 8. When the pulse returns to level "0," the voltage of the diode reverse-biased by the photo-excited carriers in response to incident light is discharged. In the initial state in which the discharge commences, the potentials of the photodiode and the drain have the characteristic illustrated in FIG. 2B. Here, Vb indicates a built-in voltage (in general, about +0.8 V) which is formed by the diode junction. Vp("0") denotes the "0" level voltage of the scan pulse, and this voltage corresponds to the design value 0.5 V-1.0 V of a polarity inverting circuit constituting the scanner. When the potential of the photodiode is increased by -0.5 V--1.0 V, the diode voltage is discharged according to the quantity of incident light, and the potential Vv' (Vv'&lt;Vv) of the diode (layer 14) diminishes (FIG. 2C). When the quantity of light further increases, the diode potential Vv" (Vv"&lt;Vv') exceeds the potential of the semiconductor body (FIG. 2D), and the electrons generated in the diode pass a portion underlying the gate of the MOST 11 and flow into the drain 15 connected with the signal output line.
On the other hand, the potential of the drain 15 remains at the potential Vv+Vb-Vp("0") throughout all of these periods because the parasitic capacitance of the vertical output line with the drains connected thereto in common is several tens times greater than the charge storage capacitance of the photodiode and is charged up to the video voltage every horizontal scan period. Accordingly, the drain or the vertical output line acts as a storage capacitance which absorbs charges that cannot be stored in the diode in consequence of the increase of the potential of the diode, and it gives rise to blooming.